This application claims the priority of German Patent Document No. 199 55 950.3, filed Nov. 19, 1999, and PCT International Application No. PCT/DE00/04037, filed Nov. 17, 2000, the disclosures of which are expressly incorporated by reference herein.
The invention relates to an antenna system for transmitting and receiving electromagnetic waves, particularly in the short-wave range. Furthermore, the invention relates to a vehicle having such an antenna system which is suitable for a mobile use.
Normally, when antennas are used in a mountainous terrain or when electromagnetic waves are transmitted, particularly in the short-wave range, a radiation of the energy is required which is as vertical or steep as possible. So-called dipole antennas are known as the ideal antennas with a radiation angle which is as vertical as possible. An ideal radiation characteristic occurs particularly in the case of a dipole antenna (abbreviated xe2x80x9cdipolexe2x80x9d) whose length corresponds to half the wavelength and which is generally called a lambda half dipole (also called xcex/2 dipole). The transmission behavior and vice versa the reception behavior of a lambda half dipole is characterized by a largely good omnidirectional characteristic and a relatively large vertical radiation angle. In the case of the lambda half dipole, it is a disadvantage that, for a particularly broad-band use in the short-wave range, for example, of from 2 to 30 MHz, the antenna length fluctuates between 75 and 5 m. Thus, such an antenna system constructed of a lambda half dipole is not suitable for a mobile use. Furthermore, dipole antennas can be used in the lower frequency range only if they are correspondingly damped. This, in turn, is connected with considerable losses which are avoided by providing matching units. However, because of the high requirements with respect to quality and matching precision, these matching units result in very high expenditures. In addition, because of the doubling of impedance values for a short dipole in comparison to the impedance values of a simple rod antenna, matching is hardly possible, so that the use of a dipole antenna for a particularly wide frequency range in the short-wave range is possible only to a limited extent and may even be impossible.
Therefore, frame or loop antennas are normally used. A frame antenna is a so-called short-circuit antenna which, in comparison to the dipole antenna, has, for the most part, good frequency behavior within a wide frequency range, in which case the radiation characteristic is limited with respect to a steep radiation angle. Because of the very high currents of the frame antenna, which are unavoidable in the operation of the antenna system, this frame antenna comprises very high-expenditure motors and electrical components which are fixedly installed. An exchange of components or a fast mounting/demounted of the entire antenna system is therefore not possible or is possible only to a very limited extent.
In European Patent Document EP 0 809 321 A1, an antenna system is described which has two dipole halves which are connected by way of a control unit. The control unit comprises a power divider module, as well as a matching module for each dipole half, as well as a central control unit which is connected with each of the two matching modules and controls them.
It is therefore an object of the invention to indicate an antenna system for a largely broad frequency range, which antenna system has a particularly simple construction, and a radiation characteristic which is as vertical or steep as possible, and is suitable particularly for a mobile use.
This object is achieved according to the invention by means of an antenna system for frequencies particularly in the short-wave range. It comprises two antennas which are mutually connected by way of a control unit and jointly form a dipole antenna. In other words, the two antennas are electrically connected with one another by means of the control unit such that they form a dipole antenna with respect to the beam characteristic. As a result, while the particularly good steep-radiation characteristic of a dipole antenna is utilized, a simple construction of the antenna system is simultaneously permitted.
Expediently, a rod antenna which is constructed as a monopole is in each case provided as the antenna. By connecting two monopoles to form a dipole antenna, on the one hand, the good steep-radiation characteristic in the short-wave range, on which the dipole antenna is based, will be utilized. On the other hand, because of the low input impedance, the use of the rod or monopole antennas permits a largely simple matching of the impedance range, whereby a particularly simple construction of the control unit is ensured.
The control unit and the two antennas are preferably arranged on a carrier element. The carrier element is preferably provided for mounting on a vehicle. A frame or a roof rack, for example, can be used as the carrier element. As a result, the antenna system is particularly easy to mount or to demount. Without being changed, the antenna system can be mounted by means of the carrier element on various vehicles independently of the vehicle type and is therefore suitable for a universal use on mobile vehicles.
Advantageously, for the radiation of a certain frequency, the two antennas are connected with the control unit such that the two antennas can be excited or fed by signals which are phase-rotated by 180xc2x0 with respect to one another. As a result of the connection of the two antennas to form a common dipole with such an antiphase relationship of the two signals, for the most part good frequency behavior is ensured while the radiation energy is simultaneously as large as possible.
In another advantageous further development of the antenna system, the control unit has a modular construction. The control unit expediently comprises at least one matching module and one power divider module. In this case, the matching module is used for matching the output of the control unit to the respective input impedance of the two antennas. By means of the power divider module, each antenna is fed symmetrically. For this purpose, a signal emitted by a transmitter is separated with a definable power by means of a power divider module; that is, half of each antenna is fed by means of the transmission power emitted by the transmitter. Such a modular construction of the control unit permits a particularly high degree of availability and independence of the antenna system. As an alternative, because of the modular construction of the antenna system, the latter can particularly easily be modified from a dipole arrangement to a simple monopole antenna. In this case, for example, one of the two antennas is deactivated by means of the control unit or is simple separated from the control unit.
In order for the antenna system can be used for a plurality of frequencies in the short-wave range, the matching module for a definable frequency comprises at least one L/C element (L element=coil, C element=capacitor). The matching module preferably includes a measuring unit and a control. By means of the measuring unit, the input impedance pertaining to the given frequency is determined. The control has the effect that, by means of the impedance values determined for the two antennas, correspondingly at least one L/C element and/or the number of so-called network elements required for the matching, such as L-elements, C-elements, is switched active. The tuning or matching operation preferably takes place iteratively for each antenna; that is, in an iterative process, the measured values determined by the measuring unit are supplied to the control for the determination of the number of L/C elements required for the matching. In each iteration cycle, the measured values and/or the number of the L/C elements are determined for both antennas, in which case the mutual dependence of the two antennas as a result of their electric coupling is taken into account. The connection of the L/C element or of the L/C elements preferably takes place synchronously for ensuring the antiphase excitation of the two antennas.
For a particularly rapid frequency change-over, the matching module expediently has a memory module in which, for various frequencies, the size and the number of L/C elements to be switched, that is, corresponding network settings, are filed. The filing preferably takes place after a concluded matching operation. For example, for a frequency range of from 2 to 30 MHz at intervals of 5 MHz, the respective pertaining impedance values and, resulting therefrom, the corresponding number, type, and/or size of the L/C elements are filed. Instead of an actual measuring of the input impedance, the corresponding combination and/or number of L/C elements will then automatically be activated by means of the control unit for a given frequency.
According to the invention, one matching module respectively is provided for each antenna. When the two antennas are identical, the respective pertaining matching module has the same construction, whereby the manufacturing costs are minimized. According to the invention, the two matching modules are connected with one another by way of a control cable. According to the invention, after an initialization phase, the two matching modules are connected with one another such that one matching module operates as a master and the other operates as a slave. In this case, the synchronization of the two matching modules takes place by way of the control cable such that the matching module operating as a master determines the required number and values of the L/C elements for the respective antenna. The matching module operating as a slave receives corresponding synchronized measuring and/or control commands from the master by means of the control cable.
For dividing the power of the radiation energy to be emitted to the two antennas, a transformer is preferably provided as the power divider module. The transformer has a particularly robust and simple construction and divides the signal coming from the transmitter with its total power into two signals with one half the power respectively and therefore with the same power.
The antenna system is preferably arranged on a vehicle, at least one device being provided which holds the two antennas predominantly in the horizontal position and diametrical with respect to one another. In other words, the free end areas of the respective antenna are arranged essentially horizontally, the two free end areas pointing away from one another at an angle of 180xc2x0.
In a further advantageous development, one matching module respectively is provided for each antenna. When the two antennas are identical, the respective pertaining matching module has the same construction, whereby the manufacturing costs are minimized. The two matching modules are expediently connected with one another by way of a control cable. After an initialization phase, the two matching modules are preferably connected with one another such that one matching module operates as a master and the other operates as a slave (sic-translator). In this case, the synchronization of the two matching modules takes place by way of the control cable such that the matching module operating as a master determines the required number and values of the L/C elements for the respective antenna. The matching module operating as a slave receives corresponding synchronized measuring and/or control commands from the master by means of the control cable.
As an alternative or in addition, the antenna system may include at least one antenna (2) which, in turn, itself is constructed as an antenna system with two antennas which are connected with one another by way of a control unit. As a result, an antenna system is provided which has a high quality and particularly good high-frequency characteristics.
The advantages achieved by means of the invention, in particular, consist of the fact that, as a result of the arrangement of two antennas each constructed as a monopole to form a dipole antenna, the particularly good beam features characterizing the dipole antenna are ensured while the construction of the antenna system is simultaneously simple. Furthermore, the largely simple matching of the two antennas permits a modular construction of the control unit, whereby a largely good flexibility and mobility of the entire antenna system as well as the exchangeability of individual components is ensured.